1. Field of the invention
This invention relates to multiple glazing units, in particular to multiple glazing units of the type comprising two vitreous material sheets positioned in a face-to-face spaced apart relationship and having a gas space there-between delimited by a peripherally extending spacer.
Multiple glazing units, for example double glazing units, are very useful for increasing thermal and sound insulation and are beneficial with regard to the sound in the interior of buildings and therefore for increasing the comfort of the occupants of the building compared to the poor insulation provided by ordinary single glazing units.
2. Description of the Related Art
Double glazing units are constituted by two sheets of vitreous material such as, glass, which are fixed and maintained in a spaced relationship with respect to one another, usually at their edges, by the intervention of a spacer. The spacer is usually a metallic profile which is adhered to the sheets, along the length of the four edges thereof. A hermetically sealed hollow space is formed between the sheets, delimited by the spacer. This space is filled with a dry gas such as dry air. A desiccant is generally associated with the spacer, in communication with the sealed hollow space in order to help maintain the gas in a dry state. It is essential that the gas confined within the space should be maintained in a dry state in order to avoid any condensation of water at the interior of the double glazing during changes in temperature. If there is condensation of water vapour on the internal walls of the sheets, the transparency of the glazing will be reduced and the visibility through the glazing will be affected.
A water tight joint is achieved with the aid of two different materials. The first material, which is highly water impermeable, but relatively flexible, is referred to generally herein as a "sealant", and may for example be a polyisobutylene. The second material which is highly adhesive and relatively rigid, is referred to generally herein as a "resin", and may for example be a polysulphide, a polyurethane elastomer or a silicone material.
A layer of sealant is positioned between the spacer and each of the sheets. A cordon of resin is positioned in contact with the sealant and extends between the sheets beyond the spacer. Alternatively, cordons of resin are positioned between the spacer and each of the sheets. Under normal conditions (at rest), while the internal pressure, that is the pressure within the gas space, is equal to the external pressure, water vapour can only enter the closed gas space of the double glazing unit, if there is a difference in partial pressure of water between the interior of the double glazing and the exterior, via the sealant between each sheet and the spacer. The sealant constitutes a barrier to the passage of humidity. Since it is a flexible material relatively impermeable to water, the humidity can therefore penetrate only with great difficulty and the small amount of water which penetrates with time is absorbed by the desiccant.
During the heating of the glazing, the internal atmosphere of the double glazing expands and the internal pressure increases. The difference between the internal and external pressures causes a force to be exerted on the sheets which tends to separate them from one another and which thereby subjects the joint to a traction stress. The resin stretches slightly and the sealant undergoes a similar expansion. If the expansion of the sealant is greater than the limit of de-cohesion thereof, the sealant ceases to be a good impermeable barrier and water can cross the joint more easily. The resin does not constitute an impermeable barrier to water; its role is to firmly maintain the two sheets in face-to-face relationship, with interposition of the spacer.
In European patent application EP-A-0534175 (Franz Xaver Bayer Isolierglasfabrik) there is described a multiple glazing unit comprising two glass sheets positioned in a face-to-face spaced apart relationship and having a gas space there-between delimited by a peripherally extending spacer. The spacer contacts the sheets and then extends slightly obliquely with respect to the inner surface of the adjacent sheet, so as to accommodate layers of butyl sealant which are positioned between the spacer and each of the sheets. Such an arrangement is intended to avoid escape of the sealant from its location to the gas space when relative movements of the sheets with regard to the spacer occur. A cordon of adhesive material is positioned in contact with the layers of sealant and extends between the spacer and each of the sheets. In the described glazing unit, the butyl sealant is disposed within a very narrow space so as to form a very narrow diffusion width to limit the passage for the ingress of humidity. However, this construction means that small movements of the glass sheets relative to each other and to the spacer result in a high elongation percentage of the sealant material, which can easily exceed its de-cohesion limit, resulting in a failure of the seal and the ingress of humidity.
Furthermore, in the described glazing unit, the above disadvantage is increased by the fact that a substantial proportion of the adhesive material extends beyond the spacer. As it is this material which serves to hold the glass sheets together against the spacer, movements of the glass sheets relative to the spacer depend on its total elongation which will be relatively high because of its large size. The total elongation of the butyl sealant in absolute terms must be equally as high and therefore the percentage elongation of the sealant can more easily exceed its de-cohesion limit, resulting in a failure of the seal and the ingress of humidity.